This invention relates to the measurement of fluid flow and, more particularly, to a method and apparatus for determining the characteristic of a flowmeter.
In order to obtain accurate readings from a flowmeter, it must be calibrated from time to time by determining its characteristic, i.e., the constant of proportionality between the flow rate of the fluid flowing through the flowmeter and the response given by the flowmeter. In the case of a turbine type flowmeter that develops electrical oscillations proportional in number to the volume of flow through the flowmeter, this characteristic would be expressed in terms of the number of pulses generated by the flowmeter per unit volume of fluid passing through the flowmeter. The flowmeter characteristic is a function of the type of fluid, as well as the fluid temperature, pressure, and flow rate, and varies as the parts of the flowmeter wear in the course of use. Apparatus to determine the characteristic of a flowmeter while in an operating fluid system is called a prover. Apparatus to determine the characteristic of a flowmeter in a special test set up, i.e., not in an operating fluid system, is called a calibrator.
A known technique for determining the characteristic of a flowmeter is to compare its response with a so-called ballistic flow calibrator or prover connected in series with the flowmeter. The ballistic flow apparatus employs a piston that travels in a cylinder in synchronism with the fluid traveling through the flowmeter. By measuring the time interval required for the piston to travel a given distance through the cylinder, an average flow rate can be calculated, which is used to determine the flowmeter characteristic. U.S. Pat. No. 3,492,856, which is assigned to the assignee of the present application, discloses improved ballistic flow apparatus in which the piston has a passage through it and a plug, i.e., a valve, that seals the passage when closed and permits fluid flow through the passage from one side of the piston to the other when open. The end of a cable wrapped around a drum is attached to the valve for the purpose of holding the piston in an upstream position prior to a test run, releasing the piston when a test run is initiated, and returning the piston to the upstream position after the test run.